At present, from the viewpoint of conservation of the global environment, a technique is desired which can efficiently utilize the energy that used to be wasted. In particular, a technique which can convert the energy that used to be wasted into mechanical energy is desired. For example, a Rankine bottoming cycle which generates steam to rotate a turbine, a Sterling cycle system using waste heat, a thermoelectric conversion system using a thermoelectric conversion element, and a thermoacoustic system converting thermal energy into sound pressure energy are known.
The thermoacoustic system uses a thermoacoustic conversion technique of exchanging energy between acoustic energy (hereinafter, also referred to as sound pressure energy) and thermal energy. The system uses a compression process and an expansion process in a fluid element of a fluid in which acoustic waves travel. Specifically, the technique uses the behavior of the compression process and the expansion process in the fluid element taking place at different locations (along the travelling direction of acoustic waves) for different longitudinal oscillations (acoustic waves).
Example thermoacoustic systems using the thermoacoustic conversion technique include a system with an apparatus having a sufficient contact area between a solid part and gas, where one end of the solid part is heated and a portion of heat is converted into acoustic energy which is supplied to the generator. The key feature of the system is the temperature gradient in the solid part of the apparatus, though the efficiency of converting energy into acoustic wave energy is disadvantageously low. The disadvantage is due to an undesirable structure and property of the thermoacoustic energy converting element part configured to convert thermal energy into sound pressure energy.
As an example of the thermoacoustic energy converting element part employing the thermoacoustic conversion technique, a thermoacoustic stack is known that is made compact to generate self-excited oscillation even with high frequency under low temperature difference (Patent Literature 1).
The thermoacoustic stack includes a plurality of through holes and is made of a material having thermal conductivity lower than 10 W/m-K. For a shorter stack length, the temperature gradient can be scaled down (proportionally reduced) so that the temperature at a high temperature side heat exchanging unit, which is required to generate a critical temperature gradient, can be reduced.